Meters, such as electricity meters, are entrusted to users who might disassemble them in order to falsify them. That is why operators and manufacturers of meters equip them with devices making it possible to detect that a meter has been opened.
An old system consists in interconnecting the structural elements of the meter that move apart when the meter is opened with an authentication seal which breaks when the meter is opened. That technique makes it possible to detect that a meter has been opened only after the event, i.e. meter opening might be detected a long time after the event, and, in addition, such detection requires a human operator to intervene.
Document EP 0 490 710 describes another known technique for automatically detecting displacement of the structural elements of the meter on meter opening. The device described includes a remanence magnetic circuit made up of at least two portions, the position of one of the portions being dependent on the position of the first element, and the position of the other portion being dependent on the position of the second element, the magnetic circuit thus being closed when said elements are in an initial position, and being open when they are in any other position. The device further includes means for exciting the magnetic circuit while it is closed and in a determined magnetic state, and control means connected to the excitation means for the purpose of controlling said excitation means. Finally, that device includes detection means suitable for producing a displacement detection signal in response to a particular variation in the state of the magnetic circuit under the effect of the excitation means, the detection means advantageously including a single-pole detection circuit.
By applying a suitable sequence of magnetizing/demagnetizing operations to the magnetic circuit, that device makes it possible to detect displacement of the structural elements of the meter automatically, regardless of whether the displacement is temporary or not, automatic detection being possible even after the event and even if the device is temporarily deprived of electricity before, during, or after meter opening.
That type of device is high in cost (compared with the overall cost of the meter), even more so since the general trend in meter prices is downward.
Document EP 549 519 describes another type of meter opening detection device. A cam is connected to one of the screws of the cover and it is rotated when the screw is unscrewed. The cam then actuates a hammer which hits a piezoelectric plate. Under the effect of the impact, the piezoelectric plate emits an electrical pulse which is recorded in a memory.
Document FR 2 681 134 describes a device for opening the housing of a meter, the device comprising a washer secured to a screw for closing the cover onto the housing. A resistive track and a conductive track for detecting the angular position of the washer are provided. The angular position of the washer is stored, and a microprocessor compares the current position of the washer with the previously stored position. Any difference between the positions indicates that the screw has been moved.
Document EP 447 615 describes various devices also using detection of a change of position of a screw connecting the cover to the housing to deduce whether the meter has been opened (or an attempt has been made to open the meter).